Diseases of the mesodermal tissues of the body including wound healing, arthritis, osteoporosis and muscular wasting diseases, affect millions of people across the globe. If existing barriers to the efficient derivation of mesodermal tissues from readily-available, patient-matched stem cells could be overcome, these diseases could be treated with stem cell-based regenerative medicine. Unfortunately, robust directed differentiation of mesoderm from embryonic stem cells (ESC) and induced pluripotent stem cells (IPSC) is currently limited to cardiovascular and blood derivatives. Reproducible protocols for the differentiation of muscle, cartilage, and connective tissues, or more specifically, those mesodermal derivatives produced solely from pre-somitic mesoderm (PSM), have yet to emerge. While it is possible to obtain some cell types of the posterior mesoderm from human ESC, these efficiencies (commonly <1%) must be vastly improved before regenerative procedures for these tissues will be feasible. Previous studies have indicated the presence of an endogenous PSM-progenitor (PSM-Pr) cell located at the border of the primitive streak and Hensen's node (the PSM-Pr niche). Progeny of the PSM-Pr colonize the paraxial mesoderm and give rise to the somites, which in turn produce the skeletal muscle, cartilage, and connective tissue of the embryo. The primary focus of the work described here is to identify and exhaustively characterize the cell types that comprise and maintain the endogenous progenitor stem cell and its niche. In this project we will (1) identify all cell types located in the border region and (2) evaluate the role of key signaling pathways in the maintenance of the PSM-Pr and the PSM-Pr niche. Successful completion of these goals will provide unprecedented insight into the generation of somitic mesoderm and enable novel approaches to the generation of therapeutically relevant mesodermal cell types for clinical use.